VHDL coding tips and tricks: sequence detector

In this article, I want to share the VHDL code for a non-overlapping sequence detector. The code is written using behavioral level modelling and state machines.

What is the system that I want to design? I assume that I have an input sequence of bits, arriving at the input port, one bit a time. Whenever the last 4 bits has the value "1011", I want a single pulse to be generated at the output port.

State Machine Diagram:

The following state machine diagram shows how this works. The VHDL code shared further down in the article is a direct implementation of this state diagram. You can see that the initial state is called "A" and as each incoming bit matches the bit in the sequence "1011", we move to the next state, finally arriving at state "D". The output pulse is generated if the next input bit is '1', matching with the right most bit in the sequence, "1011".

VHDL Code for Sequence Detector:

library ieee;
use ieee.std_logic_1164.all;

--Sequence detector for detecting the sequence "1011".
--Non overlapping type.
entity sequence_detector is
port(clk : in std_logic;  --clock signal
    reset : in std_logic;   --reset signal
    bit_in : in std_logic;    --serial bit input   
    seq_detected : out std_logic  --A '1' indicates the pattern "1011" is detected in the sequence.
    );
end sequence_detector;

architecture Behavioral of sequence_detector is

type state_type is (A,B,C,D);  --Defines the type for states in the state machine
signal state : state_type := A;  --Declare the signal with the corresponding state type.

begin

process(clk,reset)
begin
    if(reset = '1') then     --resets state and output signal when reset is asserted.
        seq_detected <= '0';
        state <= A;   --initial state
    elsif(rising_edge(clk)) then   --calculates the next state based on current state and input bit.
        case state is
            when A =>   --when the current state is A.
                seq_detected <= '0';
                if(bit_in = '0') then
                    state <= A;
                else   
                    state <= B;  --first bit matches with the 1st bit in "1011" from left.
                end if;
            when B =>   --when the current state is B.
                if(bit_in = '0') then
                    state <= C;  --2nd bit matches with the 2nd bit in "1011" from left.
                else   
                    state <= B;
                end if;
            when C =>   --when the current state is C.
                if(bit_in = '0') then
                    state <= A;
                else   
                    state <= D;  --3rd bit matches with the 3rd bit in "1011" from left.
                end if;
            when D =>   --when the current state is D.
                if(bit_in = '0') then
                    state <= C;
                else   
                    state <= A;
                    --assert pulse on output signal
                    seq_detected <= '1';   --4th bit matches with the final bit in "1011".
                end if;    
            when others =>
                NULL;
        end case;
    end if;
end process;   

end Behavioral;

I believe that the code is well commented, so I wont explain it any further. Note that, the output depends on the current state and the current input. So what we have just implemented is a Mealy state machine.

The following testbench code was written for testing the code. The sequence "11011101011" was sent to our sequence detector entity, one bit at a time starting from the leftmost bit.

What we are trying to detect is the 4 bit sequence "1011". So an output pulse should be generated on the 5th and the 11th input bit.

My input sequence: "11011101011".

Testbench Code for Sequence Detector:

library ieee;
use ieee.std_logic_1164.all;

entity tb_sequence_detector is
end tb_sequence_detector;

architecture Behavioral of tb_sequence_detector is

signal clk,reset,bit_in,seq_detected : std_logic := '0';
constant clk_period : time := 10 ns;

begin

    -- entity instantiation with named association
   uut: entity work.sequence_detector port map (
        clk => clk,
        reset => reset,
        bit_in => bit_in,
        seq_detected => seq_detected
        );

   -- generate clock
    clk_process :process
    begin
        wait for clk_period/2;
        clk <= not clk;
    end process;

   -- stimulus process : apply the bits in the sequence one by one.
    stim_proc: process
    begin       
        bit_in <= '1';             --1
        wait for clk_period;
        bit_in <= '1';             --11
        wait for clk_period;
        bit_in <= '0';             --110
        wait for clk_period;
        bit_in <= '1';             --1101
        wait for clk_period;
        bit_in <= '1';             --11011
        wait for clk_period;
        bit_in <= '1';             --110111
        wait for clk_period;
        bit_in <= '0';             --1101110
        wait for clk_period;
        bit_in <= '1';             --11011101
        wait for clk_period;
        bit_in <= '0';             --110111010
        wait for clk_period;
        bit_in <= '1';             --1101110101
        wait for clk_period;
        wait;        
    end process;

end Behavioral;